1. Field of the Invention
The present invention relates to viewfinder optical systems and, more particularly, to a viewfinder optical system of the external type independent of a photographic optical system, in which an inverted real viewfinder image (object image) formed by an objective lens system (objective optical system) on a primary image plane is converted into a non-inverted erecting viewfinder image by using a field lens and an image inverting means so as to be observed through an eyepiece lens, i.e., the viewfinder optical system of the real image type suited to still cameras, video cameras, etc.
2. Description of Related Art
Heretofore, as a type of viewfinder optical system for photographic cameras, video cameras, etc., there are a variety of proposals for a viewfinder optical system of the real image type in which a real viewfinder image formed on a primary image plane is observed through an eyepiece lens after passing through a field lens and an image inverting means. Such a real-image type viewfinder optical system is recently used in a great number of cameras, because it is easy to minimize the size of the entirety of the viewfinder optical system.
A viewfinder optical system of the real-image type, as disclosed, for example, in Japanese Laid-Open Patent Applications No. Hei 1-154114 and No. Hei 3-121408 is constructed in simple form, as comprising an objective lens system composed of only one lens of positive refractive power, a field lens of positive refractive power, an image inverting means and an eyepiece lens.
Further, in another Japanese Laid-Open Patent Application No. Sho 63-226616, there is proposed a viewfinder optical system having a simple construction, which comprises an objective lens system composed of two, positive and negative, lenses, an image inverting means composed of two, first and second, prisms, and an eyepiece lens, in which the objective lens system is made to form an object image in between the first prism and the second prism, and an entrance surface of the first prism is made to have a refractive power, thus the first prism functioning also as a field lens.
In the conventional real-image type viewfinder optical system, if the number of lens elements in the objective lens system is reduced to one, the lens configuration can be simplified, but the optical performance becomes difficult to keep good.
In particular, when it is designed to widen the field angle of the viewfinder optical system, problems arise in that it becomes difficult to correct distortion and that the magnification of the viewfinder optical system becomes low.
Further, in the viewfinder optical system having such a construction that the image inverting means is composed of two, first and second, prisms to effect an image inverting function and an entrance surface of the first prism is made to have a refractive power to function as a field lens, there are problems that, because the field lens is far away from an image plane of the objective lens system, the action as a strong field lens is difficult to obtain and that the first prism comes to increase in size.